Many industrial applications require large scale generation of heat from burners for process heaters, boilers, furnaces, or other fired heating systems. If the burner fuel is thoroughly mixed with air and combustion occurs under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, e.g., at a high flame temperature, nitrogen present in the combustion air reacts with oxygen to produce nitrogen oxides (NOx). Other conditions being equal, NOx production increases as the temperature of the combustion process increases. NOx emissions are generally considered to contribute to ozone depletion, acid rain, smog, and other environmental problems.
For gaseous fuels with no fuel bound nitrogen, thermal NOx is the primary mechanism of NOx production. Thermal NOx is produced when the flame reaches a high enough temperature to break the covalent N2 bond so that the resulting “free” nitrogen atoms then bond with oxygen to form NOx.
Typically, the temperature of combustion is not great enough to break all of the N2 bonds. Rather, most of the nitrogen in the air stream passes through the combustion process and remains as diatomic nitrogen (N2) in the combustion products. However, some of the N2 will typically reach a high enough temperature in the high intensity regions of the flame to break the N2 bond and form “free” nitrogen. Once the covalent nitrogen bond is broken, the “free” nitrogen is available to bond with other atoms. Fortunately, the free nitrogen will most likely react with other free nitrogen atoms to form N2. However, if another free nitrogen atom is not available, the free nitrogen will react with oxygen to form NOx.
As the temperature of the burner flame increases, the stability of the N2 covalent bond decreases, causing increasing production of free nitrogen and thus also increasing the production of thermal NOx emissions. Consequently, in an ongoing effort to reduce NOx emissions, various types of burner designs and theories have been developed with the objective of reducing the peak flame temperature.
The varied requirements of refining and petrochemical processes require the use of numerous different types and configurations of burners. The approaches used to lower NOx emissions can differ from application to application. However, thermal NOx reduction is generally achieved by slowing the rate of combustion. Since the combustion process is a reaction between oxygen and the burner fuel, the objective of delayed combustion is typically to reduce the rate at which the fuel and oxygen mix together and burn. The faster the oxygen and the fuel gas mix together, the faster the rate of combustion and the higher the peak flame temperature.
Examples of different types of burner design approaches used for reducing NOx emissions have included:                a. Staged air designs wherein the combustion air is typically separated into two or more flows to create separate zones of lean and rich combustion.        b. Designs using Internal Flue Gas Recirculation (IFGR) wherein some of the burner fuel gas passes through and mixes with the inert products of combustion (flue gas) in the combustion system to form a diluted fuel gas which burns at a lower peak flame temperature.        c. Staged fuel designs wherein fuel gas is separated into two or more flows to create separate zones of lean and rich combustion.        d. Designs using External Flue Gas Recirculation (EFGR) wherein inert products of combustion are mixed with the combustion air to reduce the oxygen concentration of the air stream supplied to the burner, which in turn lowers the peak flame temperature.        e. Designs using “flameless” combustion wherein most or all of the burner fuel gas passes through and mixes with inert products of combustion to form a diluted fuel gas which burns at a lower peak flame temperature. The mixture of fuel gas and inert products of combustion can be as high as 90% inert, thus resulting in a “transparent” flame.        f. Designs using steam and/or inert injection into the burner fuel gas wherein the steam or inert components mix with the fuel gas so that the resulting composition will burn at a lower peak flame temperature.        g. Designs using steam and/or inert injection into the combustion air stream wherein the steam and/or inert components mix with the combustion air so that the resulting composition will burn at a lower peak flame temperature.        